1. Field of the Invention
This invention relates to a hand held, electronic grading level comprising an ultrasonic ranging (i.e. sonar) system and a telescopic sight level for use by a grading engineer or grade checker to automatically and accurately compute the grade of a ground plane and indicate whether said ground plane is level or within accepted tolerance limits.
2. Prior Art
As is known to grading engineers, grade checkers, surveyors, and the like, an "eyeball" approach is commonly used to determine the grade and level of land under construction. More particularly, an engineer's stake is hammered into one end of the land and a colored ribbon, or the like, is tied to said stake to serve as a reference sight mark at an arbitrary elevation. The grade checker then stands at the opposite end of the land and holds a plumb string to his eye while attempting to horizontally align his thumb with the target ribbon that is tied to the stake. In the alternative, a hand held peepsight is horizontally aligned with the target ribbon and a (e.g. six foot) ruler is used to measure the distance of the peepsight above the ground. By comparing the distance along the string or ruler with the elevation of the ribbon along the stake, an indication is obtained as to the grade of the ground plane located between the engineer's stake and the grade checker.
The foregoing "eyeball" approach is not always accurate, particularly when the ground plane to be checked is long and the grade checker is located far away from the engineer's stake. Therefore, inaccuracies are often introduced into the grade check which may result in the ground being overfilled or undercut. Such inaccuracies often occur as a consequence of the checker making erroneous mathematical calculations in his head and/or reading the ruler incorrectly or upside down, whereby to obtain a reverse grade. Obviously, the cost, lost time and waste of materials that can result from such miscalculations and inaccuracies can be extremely burdensome to a construction site that is under tight budget constraints, even if the grade error is relatively small.
It would therefore be desirable to have an apparatus by which to automate the method for checking grade, whereby to increase the accuracy of measurement and thereby avoid the expensive and time-consuming errors that are sometimes associated with the "eyeball" approach.